Aim: Traditionally, South American aquatic insects have been divided into cold and warm adapted forms. Cold-adapted forms inhabit freshwater systems from higher latitudes, or higher altitudes even around the Equator. Warm-adapted groups are defined as those found in lower latitudes and altitudes. This work aims to answer the questions: Are mayfly assemblages geographically segregated according to geographical (latitude) and topographical (altitude) surrogates of temperature? If so, where is this transition located?Location: South America. Methods:We compiled a data set about the relative incidence of 52 mayfly genera in 326 sampled communities. They span from 0 to 4,320 m and from 47.77°S to 5.74°N latitude. By virtue of the compositional nature of the data set, we applied the statistical procedures behind the Aitchison compositional data analysis. We delimited groups of assemblages based on their Aitchison distances and projected the data points onto a biplot obtained through Principal Component Analysis adjusted to compositions (Aitchison PCA). Results: A strong correspondence among biological and geographical information was detected, with mayfly assemblages clearly segregated in space. Andesiops and Meridialaris are typical cold-adapted forms; Baetodes, Leptohyphes and Thraulodes represent the warm group. Thermal groups can be separated by a curved line of altitude in function of latitude expressed in terms of a superellipse arc. Main conclusions: The classical ecological bipartition of mayflies into warm and cold freshwater groups is formalized quantitatively. The dividing line between warm and cold assemblages levels off at high altitudes (c. 3,300 m) around the Equator and falls to sea level at southern latitudes. The community bipartition line is useful for tracking global change through records of altitudinal displacement below and above of the warm/cold line of involved ecological groups.

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Pine afforestation of grassland streams may lead to changes in species traits and therefore functional diversity of epilithic algal community. Here, we studied trait-based responses in three grassland and three afforested streams in a mountain watershed of Córdoba, Argentina. We hypothesized that afforestation would reduce functional diversity through a simplification of periphyton architecture resulting from reduction in light availability, and that changes in hydrological periods would influence community responses. Algal samples were collected at each stream during two different hydrological periods (high flow and low flow), and physicochemical variables were recorded. Selected traits included strategies and morphological characters related to resource access and disturbance resistance (size, morphological guild, resource requirement, attachment mechanism and life-form). We calculated two indices of functional diversity: Rao's quadratic entropy (FD Q ) and functional variance. Most trait categories showed a significant effect of one or both factors; 26 % discriminated between vegetation types, 26 % reflect the changes between hydrological periods, and 47 % were sensitive to both of them. Our results revealed some categories of traits that can be used to distinguish changes in riparian vegetation, such as unicellular life-form and high-profile guild. Functional diversity of single traits was affected differently by pine afforestation. However, the most integrative index, the FD Q mean, partially supported our hypotheses. Afforestation reduced FD Q mean by 50 %, but only during low-flow period. FD Q mean was high and similar between streams at high flow, when environmental factors, such as discharge and temperature, could prevail on differences in riparian vegetation.

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